(a) Field of the Invention
This invention relates to a method of making a lead-calcium-aluminum alloy at relatively low temperatures and without resorting to use of inert gases or fluxes.
(b) State of the Art
Aluminum is often added to lead-calcium and lead-calcium-tin alloys to prevent oxidation of the calcium during remelting of the alloy and subsequent casting and handling of the molten metal. Such use of aluminum in lead-calcium-tin alloys is described in U.S. Pat. No. 4,125,690.
A common method of alloying aluminum into lead entails melting and heating the lead to a temperature above the melting point of aluminum (660.degree. C.). At this temperature the aluminum melts and becomes alloyed with the lead readily with some loss due to oxidation. At temperatures below the melting point of aluminum an external adherent oxide skin prevents the aluminum from dissolving in the lead even though it is soluble in small amounts. Therefore aluminum and lead cannot be effectively alloyed at temperatures below 660.degree. C.
Calcium is generally alloyed into lead under an inert gas or molten salt cover to prevent oxidation. High temperatures are required to keep the salt cover molten or to effect complete dissolution of Pb.sub.3 Ca compounds into the lead. By means of this procedure a master alloy of 1-2% calcium is normally produced. The master alloy is then added to lead or lead-aluminum alloy to produce the final alloyed product.
Several problems are associated with the current approach to alloying calcium and aluminum into lead. First, the kettles used in alloying the lead must be heated to temperatures above 660.degree. C. to permit efficient addition of aluminum. This dramatically reduces the life of the alloying kettle. In addition recovery of calcium in making the 1-2% master alloy is generally less than 90% because of oxidation of the calcium during alloying and pouring despite the inert gas and salt covers. Finally, because of the limited solubility of aluminum in lead, it is not possible to directly alloy the aluminum into the calcium-lead master alloy.
A new direct method of alloying calcium and aluminum with lead alloys has now been discovered. The method avoids the use of inert atmospheres or flux covers; gives nearly 100% recovery of calcium and aluminum and is operative at low temperatures where damages to alloying kettles is negligible. Moreover, because of the lower temperature requirements, fuel requirements are reduced.